1. Field of the Invention
This invention relates to the use of photolithographic masks for selectively exposing portions of photosensitive materials; and, more particularly, to the formation and use of integral spacers on such masks for holding the masks in substantially uniform proximity with the photosensitive material during the selective exposure.
As is well known, photolithographic processes are conventionally used in the fabrication of semiconductor devices and thin film devices. In such processes, a photolithographic mask, hereinafter termed simply a "mask", typically includes either opaque image portions on an otherwise transparent background or transparent image portions on an otherwise opaque background, depending upon whether a positive photoresist or a negative photoresist coating is to be employed on the article to be processed.
The term "positive" photoresist is used to designate that type of photosensitive material having the characteristic that exposure to light of suitable wavelength allows the exposed portions to be subsequently removed or developed away. "Negative" photoresist is the opposite type photosensitive material, wherein those portions which are exposed are the portions which are retained after development.
Early photolithographic processes typically employed "contact" printing wherein the mask was simply pressed into intimate contact with the photosensitive coating and light was directed through the mask to selectively expose the coating. (See, e.g., Tong, "Mack Manufacture for Integrated Circuits," The Western Electric Engineer, Dec. 1967, pp. 34-43.) Contact printing, while eminently suitable for sharp-line definition, suffers the problem that masks themselves are often not perfectly flat, but more seriously from the problem that semiconductor wafers containing photosensitive coatings typically have spikes protruding therefrom or dirt particles thereon which are sufficiently hard to scratch either the mask or the photosensitive coating itself. Because scratches on the mask and/or scratches in the photosensitive coating deleteriously affect the quality of the selective exposure, such scratching can rapidly destroy the usefulness of the mask and can also degrade the yield of the articles being processed.
To prevent scratching of the photomasks and of the photosensitive layers on the articles being processed, it has become conventional to employ proximity printing, wherein the mask is held near to, but uniformly spaced from, the photosensitive coating to accomplish the selective exposure. Considerable time, effort and money has been spent in developing complex and still, in many ways, unsatisfactory equipment to sufficiently reproducibly achieve the requisite very close spacings, e.g., from less than a mil (0.001 inch) to a few mils, typically used in proximity printing.